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Publication Number: FHWA-HRT-04-150
Date: July 2006
The petrographer can examine any specimen(s) of concrete the client wishes to submit for his or her scrutiny; however, unless the petrographer has been informed of any problem(s) that the concrete has developed and how the specimens were collected relative to the location of any problem areas, the examination may not yield any meaningful information. Complete documentation describing the placement and any problems concerning it must accompany the specimens.
Unless samples of concrete are obtained according to a statistically based sampling plan (see ACI 201.1R; ASTM C 42; ASTM C 823, "Sampling Concrete in Constructions"; ASTM C 856, "Samples"), the results of any examination or testing cannot be considered to apply to any portion of the HCC not thus sampled. Information concerning the statistical sampling of concrete and concrete-making materials can be found in Abdun-Nur and Poole (1994) and Steele (1994). The bibliographies and appendix of these references provide source material to cover most sampling problems.
The specimens for petrographic examination may be of several types: (1) cores (drilled from the hardened concrete with a diamond core drill); (2) cylinders cast from the unhardened mixture at the time the HCC was placed; (3) fragments broken naturally or by sledge or pneumatic hammer from the placement; and (4) laboratory specimens, such as mortar bars, beams, and test cylinders. It is important that the concrete in the specimens be as nearly like the HCC under investigation as practical.
When the specimens of HCC submitted to the petrographer are insufficient in number, size, depth, or distribution of the source locations, they must be treated as preliminary specimens that are to be examined to determine the necessity for a more complete examination of the placement and a more extensive sampling program.
Despite the fact that most clients would prefer to take specimens of only the most questionable area (often an area they wish to remove anyway), the petrographer must become familiar with the material of the entire placement. For example, if one portion of a placement is showing distress or exhibits failure of some sort, specimens should be obtained not only from the area of failure, but also from nearby HCC that is presumably of the same mixture, but that is free of failure. These companion specimens should be sufficiently large and numerous to represent the "healthy" condition. They should be composed of the same materials (aggregates, cement, and admixtures) and should have been specified to have been made from the same mixture proportions. In addition, if various degrees of failure of the material exist, the specimens submitted must also represent these intermediate conditions.
The steps taken to develop a sampling plan should include the following:
The client should furnish the petrographer with complete information concerning the sampling plan used and the sketch showing the relationships between the specimens. Specimens must be labeled so that their source location can be identified.
The results of any testing already performed on the concrete in question, the data collected, their relationship to the specimens submitted, and the reasoning used in selecting the locations sampled (see ASTM C 856, "Samples") should be reported to the petrographer.
The location from which the samples are obtained will depend on the objectives of the investigation. Specimens of concrete should be as little damaged by the removal methods as possible; otherwise, the petrographer will not be able to ascertain which cracks are indigenous to the HCC of the placement and which were caused by the collection procedures. Core specimens are usually preferred.
Air-void determinations may be required whenever it is suspected that the air-void content is not sufficient to provide protection from freezing and thawing deterioration or whenever it is suspected that the cause of low strength might be excess air content. If the air-void content of the entire placement is in question, sampling should follow the instructions detailed in ASTM C 457, "Sampling," as follows:
To determine the compliance of hardened concrete with the requirements of the specifications on the air-void content or the specific surface and spacing factor of the void system, a sample of the concrete should be obtained from at least three locations in the body of the concrete and measurements should be taken using a microscope on at least one section prepared from each of at least three such samples.
The three locations sampled must be selected from the entire body of the placement under study according to a rigorously random plan without regard for areas of extreme deterioration. The areas exhibiting specific features should be sampled separately. These sampling guidelines may be followed for any concrete suspected to deviate from the required quality. Each sample should be large enough to allow the petrographic staff to prepare at least the minimum area of finished surface given in ASTM C 457, table 1. The petrographer should be consulted in any case of doubt.
Core specimens of overlays that have cracked must be taken with special care. All cores must be the full depth of the overlay. Each core should be centered on a crack and should be examined as it is removed from the placement. If the crack extends to the bottom of the core, subsequent cores should be deep enough to include the full depth of the crack system.
Core specimens of delaminated overlays must include at least 50 mm of the substrate concrete. If the core comes apart at the bond line during coring, the two pieces will grind on each other and will destroy the evidence of the nature of the bond. In this event, additional cores should be taken in an effort to obtain specimens of the bond itself.
If freezing of the HCC while fresh is suspected, at least one specimen should be obtained from the edge of the placement, from up against the form or from a place exposed to the ambient temperature. It is in such an exposed area that the casts of ice crystals will form first. If companion cylinders were cast and cured as was the placement, they may show ice crystal casts on the surface in contact with the mold. An ambient freezing temperature while the concrete is fresh usually affects the wearing surface only if the curing material is insufficient to retain the heat generated by the hydration of the cement or if the curing material is prematurely removed (possibly by wind).
Unusual conditions may necessitate unusual methods of sampling. For example, the giant popout (shown in figure 203) was found lying loose on a railroad tie beneath a concrete highway bridge. It was a curiosity, and we were concerned only with the reason for the popout and not with the main mass of the concrete. A method of reaching the spot on the overhead concrete was found, and the hygroscopic glass shown in figure 204 was recovered from the matching depression. The ordinary popouts (photographed for size contrast in figure 203) were recovered loose from a highway surface. Each of these contained a fragment of porous chert at its apex. These may be considered classic popouts, pushed out of the pavement surface by the freezing and expansion of water in the porous chert. These specimens are useful reference specimens, and the chert popouts are sufficient evidence to allow the petrographer to recommend against further use of this particular aggregate in wearing courses; however, the sampling procedures, although sufficient, are hardly those classically specified.
Figure 203. Giant popout caused by a piece of glass (accompanying it are several small popouts of a more usual size caused by porous chert particles).
Figure 204. Glass particle.
It is important that the field sampling of aggregate specimens be such that the aggregate sample is truly representative of the material proposed for use and that the ratios between the various lithologies and sizes have not been influenced by the sampling procedures (Landgren, 1994; Galloway, 1994).
Access to laboratory-produced specimens of HCC and the mixture proportions by which they were fabricated can prove to be very useful to the concrete petrographer. They provide an excellent opportunity for the study of specimens of HCC produced with various experimental materials. They can also provide examples of HCC produced with a large variety of aggregate and numerous different admixtures so that the variations among the specimens can be correlated with known differences in the design of the mixture. Various curing methods under various conditions and at various degrees of maturity can also be studied. The results of the petrographic observations can be compared with the data obtained in the laboratory. The data obtained from the concrete mixing laboratory and made available to the petrographer should include the exact nature and source of the ingredients, the proportions of the mixture, and the results of any testing, such as the following:
The concrete mixing laboratory and the concrete petrographic laboratory supplement each other. When the results of the testing done in the concrete mixing laboratory do not seem to make sense or do not explain the problem under consideration, petrographic examination may be able to provide illumination.
Construction problems usually require rapid solutions that cannot wait for results from long laboratory procedures. Waiting until a laboratory mixture is prepared, cured, and tested so that the resulting concrete can be compared with the concrete at a particular problem site is often not possible. Such experimentation must usually be performed later under more deliberate, controlled conditions. In any case, it is widely recognized that it is difficult to duplicate bad concrete in the laboratory. Features that are a result of poor workmanship or incomplete mixing are especially difficult to duplicate. This may be partially caused by the difference in size between a ready-mix truck and a laboratory mixer and partially caused by the natural reluctance of laboratory-trained concrete technicians to violate normal procedures. Particular difficulty may be found when trying to duplicate problems that have been caused by field alteration of the mixture at the construction site. It is all too common that water wasadded to the mixture after 253 the concrete began to stiffen and after the air-content determinations were made. This retempering (see appendix C) is usually not documented and must be inferred from the parameters of the concrete (Erlin, 1994; Mielenz, 1994).
Investigations that include the fabrication of special concrete mixtures are really research projects, but must often be undertaken before a truly informed opinion can be made about the quality, the cause of the particular features, or the reason for the failure of HCC from particular construction site
Abdun-Nur, E.A., and Poole, T.S. 1994. "Techniques, Procedures, and Practices of Sampling of Concrete and Concrete-Making Materials," Significance of Tests and Properties of Concrete and Concrete-Making Materials, P. Klieger and J.F. Lamond, Eds., Report No. ASTM STP 169C. American Society for Testing and Materials, West Conshohocken, PA, pp. 15-22.
American Concrete Institute. ACI 201.1R: Guide for Making a Condition Survey of Concrete in Service, ACI Manual of Concrete Practice: Part 1, Materials and General Properties of Concrete. Farmington Hills, MI.
American Society for Testing and Materials. ASTM C 42: Standard Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete, Annual Book of ASTM Standards: Volume 04.02, Concrete and Aggregates. American Society for Testing and Materials, West Conshohocken, PA.
American Society for Testing and Materials. ASTM C 457: Standard Test Method for Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete, Annual Book of ASTM Standards: Volume 04.02, Concrete and Aggregates. West Conshohocken, PA.
American Society for Testing and Materials. ASTM C 823: Standard Practice for Examination and Sampling of Hardened Concrete in Constructions, Annual Book of ASTM Standards: Volume 04.02, Concrete and Aggregates. West Conshohocken, PA.
American Society for Testing and Materials. ASTM C 856: Standard Practice for Petrographic Examination of Hardened Concrete, Annual Book of ASTM Standards: Volume 04.02, Concrete and Aggregates. West Conshohocken, PA.
Erlin, B. 1994. "Petrographic Examination," Significance of Tests and Properties of Concrete and Concrete-Making Materials, P. Klieger and J.F. Lamond, Eds., Report No. ASTM STP 169C. American Society for Testing and Materials, West Conshohocken, PA, pp. 210-218.
Galloway, J.E. 1994. "Grading, Shape, and Surface Properties," Significance of Tests and Properties of Concrete and Concrete-Making Materials, P. Klieger and J.F. Lamond, Eds., Report No. ASTM STP 169C. American Society for Testing and Materials, West Conshohocken, PA, pp. 401-410.
Landgren, R. 1994. "Unit Weight, Specific Gravity, Absorption, and Surface Moisture," Significance of Tests and Properties of Concrete and Concrete-Making Materials, P. Klieger and J.F. Lamond, Eds., Report No. ASTM STP 169C. American Society for Testing and Materials, West Conshohocken, PA, pp. 421-428.
Mielenz, R.C. 1994. "Petrographic Evaluation of Concrete Aggregates," Significance of Tests and Properties of Concrete and Concrete-Making Materials, P. Klieger and J.F. Lamond, Eds., Report No. ASTM STP 169C. American Society for Testing and Materials, West Conshohocken, PA, pp. 341-364.
Steele, G.W. 1994. "Statistical Considerations in Sampling and Testing," Significance of Tests and Properties of Concrete and Concrete-Making Materials, P. Klieger and J.F. Lamond, Eds., Report No. ASTM STP 169C. American Society for Testing and Materials, West Conshohocken, PA.